1. Field of the Invention
In thermoplastic extrusion operations requiring the feed of molten polymer from a single extruder to a pair of extrusion outlet orifices or dies, care must be taken to insure that the distribution and pressure of the molten polymer exiting from each die is easily regulatable within precise limits to insure product quality and uniformity of the individually extruded films.
2. Description of the Prior Art
In the past, it has been common practice to feed dual tubular extrusion dies with a single extruder (not shown) as illustrated in FIG. 1. When it is desired to adjust the average film thickness 11 of the respective tubes, this may be accomplished by varying the rotational speed of the positively driven take-up nip rollers 12. An increase in nip-roller speed draws the still semi-molten polymer immediately adjacent the extrusion orifice away from the die 13 at a higher rate, resulting in a gauge reduction of the final tubular product. Such an arrangement for gauge control adjustment can be employed only when the tubes are being fed to separate, in-line, bag making machines due to the differential in the rate of travel between the flattened tubes.
FIG. 2 shows an alternate prior art technique of forming two tubes from a single extruder (not shown) by extruding a relatively large tube 14, passing the extruded tube through collapsing shields 15, nip rollers 12, and subsequently slit-sealing the tube utilizing a heated wire or blade 16. The forming technique has the advantage of permitting two tubular streams to be fed through one set of wide downstream equipment, such as printing presses and bag-making machines, but it has the drawback of introducing an additional heat seal seam in the finished bag product making such bags more susceptible to rupture under load stresses applied during use of the bag.
It would be possible to attain the quality advantage of the dual tube technique (FIG. 1) in which there is no side seal, along with the economic advantage of the slit-seal technique (FIG. 2) in which there is only a single set of downstream machinery, by extruding two tubes and running them through the same set of nip rolls. However, this does not permit operating the two tubes at different take-off speeds in order to adjust their relative average gauge. Such adjustment would require the employment of a separate flow-control valve for each die.
A typical flow control valve that is commonly used in prior art is shown in FIG. 6. Molten thermoplastic material 203 is caused to flow through gap 201, where it is subjected to a pressure drop. As the valve is adjusted by moving restricter 202, gap 201 is changed, and the resulting pressure drop is changed accordingly. If there are two such valves in parallel, fed from a common manifold source and discharging into identical downstream pressures, the flow rate through the two valves is proportional to approximately the 5th power of the size of the gap. Since this gap is usually a small fraction of an inch, it may be seen that a very small change in the size of this gap will result in an undesirably large change in flow rate. For example, if the gap is 0.1 inch, and it is changed by a very small amount such as 0.005 inches, there will be a resulting change in flow through that gap of approximately 28 percent, provided the supply and discharge pressures remain unchanged. Precise flow rate control with such valves is, at best, extremely difficult. It may be seen that prior art valving means such as that described above, when used in a pair (one for each die) cannot readily be used for the purpose of accurately balancing the flow, because they have the following limtations:
(a) They are not sufficiently precise to permit very accurate adjustment of flow distribution, for the reasons described above.
(b) Adjustment of one valve results in a change in net back pressure against the extruder, thus causing a change in total flow leaving the extruder. Therefore, any adjustment of a valve would necessitate re-adjustment of the extruder speed in order to restore the desired overall average thickness of the two streams.
(c) Since there is a separate valve for each die, the operator is required to make a decision as to which valve to adjust. After numerous adjustments, this sometimes results in both valves being almost fully closed, while at another time both may be almost fully open, so there is a difference in operating conditions (back pressure) for the extruder from one time to another.